An engineering perspective of ceramics applied in dental reconstructions

Abstract The demands for dental materials continue to grow, driven by the desire to reach a better performance than currently achieved by the available materials. In the dental restorative ceramic field, the structures evolved from the metal-ceramic systems to highly translucent multilayered zirconia, aiming not only for tailored mechanical properties but also for the aesthetics to mimic natural teeth. Ceramics are widely used in prosthetic dentistry due to their attractive clinical properties, including high strength, biocompatibility, chemical stability, and a good combination of optical properties. Metal-ceramics type has always been the golden standard of dental reconstruction. However, this system lacks aesthetic aspects. For this reason, efforts are made to develop materials that met both the mechanical features necessary for the safe performance of the restoration as well as the aesthetic aspects, aiming for a beautiful smile. In this field, glass and high-strength core ceramics have been highly investigated for applications in dental restoration due to their excellent combination of mechanical properties and translucency. However, since these are recent materials when compared with the metal-ceramic system, many studies are still required to guarantee the quality and longevity of these systems. Therefore, a background on available dental materials properties is a starting point to provoke a discussion on the development of potential alternatives to rehabilitate lost hard and soft tissue structures with ceramic-based tooth and implant-supported reconstructions. This review aims to bring the most recent materials research of the two major categories of ceramic restorations: ceramic-metal system and all-ceramic restorations. The practical aspects are herein presented regarding the evolution and development of materials, technologies applications, strength, color, and aesthetics. A trend was observed to use high-strength core ceramics type due to their ability to be manufactured by CAD/CAM technology. In addition, the impacts of COVID-19 on the market of dental restorative ceramics are presented.

in the perpendicular direction. 17,18 Due to its intrinsic nature, enamel presents a stress-strain response similar to some metals, enabling its function throughout the life of an individual. 19 Dentin has a fracture toughness ranging from 1 to 2 MPa.√m in the perpendicular and parallel directions to the tubules. 20,21 The complex nature of the dentin-enamel junction (DEJ) has proven to be extremely relevant since it presents a hierarchical microstructure that stops cracks and reduces stresses in enamel as a graded elastic modulus layer. 22 Detailed work describing the unique structure, micromorphology, and mechanical properties of human teeth and the DEJ can be found elsewhere. 23,24 Ultimately, from a functional biomechanical perspective, human teeth biomimetics makes material development and application techniques challenging to this date. Table 1 shows the values of translucency and mechanical properties of the most currently used dental ceramics.
Although the currently available materials can mimic the gradual change of color and opacity of natural teeth, the microstructure remains a challenge.
It would be necessary to build the enamel, DEJ, and dentin the way they are naturally to design a material that resembles human teeth in appearance, mechanical properties, and structural architecture.
Ameloblasts form the enamel arranged in a close, overlapping way, forming three zones: inner enamel zone, enamel decussation zone, and enamel parallel prism zone. DEJ is the interphase between enamel and dentin, formed with the alignment of ameloblast and odontoblast with approximately 60-100 μm width of the graded structure. At last, dentin has a structure with tubules that course from the DEJ to the pulp radially inward. Additive manufacturing offers a wide range of possibilities to fabricate materials based upon natural tissues, such as enamel, dentin, and the DEJ, to obtain an overall structure. Therefore, holistic teeth biomimetics remains a challenge for future technologies. 22,34 Among all the available materials, dental ceramics have played an essential role in many fields, such as implants, orthodontics, and prosthodontics. 35 The preference for ceramics is due to their biocompatibility, aesthetics, durability, and tailored design. 36 Adding to this, the capacity to have proper translucency, strength, outstanding wear resistance, and intraoral stability make ceramics suitable for routine use in dentistry. In other words, all these properties make this material suitable for various restorative applications. 37 A large variety of ceramics have been used in the dental field according to the restoration type. The two major restoration types are Ceramic Metal System and All-Ceramics, as seen in Figure 1. Their mechanical and optical properties can be adjusted based on their compositions, type of application, and fabrication methodologies. This study reviews the use of dental ceramics to restore or replace damaged teeth. We will discuss the main factors that influence the performance of the restoration, such as the mechanical properties, fabrication methodologies, design, aesthetic aspects, and applicability.

Ceramic-metal system
Ceramic-metal system, also known as porcelainfused-to-metal (PFM), was the first configuration used for fixed dental prosthesis fabrication in the early 60s, becoming a well-established treatment approach. 39 The benefits of these restorations include longevity, strength, and stability of the underlying metal framework that can withstand heavy mastication forces. 40  anterior and posterior Fixed Dental Prostheses (FDP), PFM still presents higher survival rates, especially for implant-supported in long-span prostheses, when compared with porcelain fused to zirconia. 41,42 PFM crowns are widely indicated in oral rehabilitation.
The metal alloy core may present several compositions (see ADA dental alloy classification) and is fabricated using different techniques, including casting, subtractive manufacturing or milling, and additive manufacturing or 3D printing. 43-45 Furthermore, the opaque metal substructure is veneered with feldspathic ceramic using conventional and contemporary technologies such as sintering, computer-aided design, computer-aided manufacturing, and heat pressing. 46 Failure occurrence due to chipping fracture is one of the main concerns when applying PFM restorations.
Chipping fracture is usually limited to the veneering ceramic layer due to the lower fracture toughness (K Ic ) relative to the framework material. One reason for the higher rate of chipping in porcelain fused to zirconia (PFZ), when compared with PFM restorations, is that the virtual absence of the leucite reinforcement in PFZ, tailored to match the coefficient of thermal expansion (CTE) with the zirconia core, results in an approximately 50% decrease in the K IC of the veneering porcelain. 47 When chipping occurs, the core material usually remains unexposed since it is covered with a thin veneer ceramic layer on its surface. Also, the CTE mismatch between the veneering and the framework material is an essential source of residual stresses.
This mismatch can be beneficial when framework's CTE is higher than veneer's CTE. If metal contracts slightly more than the porcelain upon cooling from firing to room temperature, it leaves the porcelain in residual compression. The propagation of potential cracks in the porcelain material is suppressed by these compressive stresses, increasing the ceramic fracture resistance. 48 Table 2 shows some of the values of CTE of the metal framework and ceramic veneer. Tanaka,et al. 52 (2019)   to the residual compressive stresses that can increase the load necessary to initiate the median cracks or modify the crack growth velocity.
The manufacturing method can also influence material properties. Zhou,et al. 53 (2019)  Given these results, it was suggested that the CAST method presented the best dynamic combination of CTE with the porcelain. Based on the mechanical characterization and microstructural evaluation, the CAST group was better than the SLM group.
Considering the aesthetic aspect, an ideal restoration should match the contour, color, surface texture, fluorescence, translucency, and opalescence of natural teeth. 55 Metal-ceramic restorations have been extensively used in restorative dentistry due to their high fracture strength. It still represents the material with the widest indications in oral rehabilitation and has a precise use to mask darkened substrates such as titanium implant abutments or darkened teeth. 12 However, the metal substructure prevents light transmission and makes it challenging to achieve an acceptable masking effect. 12,55

All ceramic
Traditional dental ceramics

Silica-based ceramics
Silica-based ceramics were the first materials used in dentistry to make porcelain prostheses. 56 Also called feldspathic ceramics, this composition belongs to traditional ceramic materials widely used in all-ceramic restoration. This material is classified as porcelain-based because it undergoes a vitrification process in which numerous crystalline cores are surrounded by a silica-based glassy matrix. 57 Such microstructure is formed due to the high temperature used to process the raw material composed of silica-based ceramics.
The main composition of this traditional dental ceramics is 70 -75% of potash feldspar as crystalline phase, and the remainder of kaolin (Al 2 O 3. 2SiO 2 .2H 2 O) as a binder. 58 Since quartz is not a strong material, Al 2 O 3 is added to improve mechanical performance. 57 Also, a few amounts of pigments can be used to reach different ranges of opacity and translucency, producing different tooth shades. 59 Silica-based ceramics can mimic the shades of a natural tooth, making them suitable for veneer inlays and onlay restorations. 38 However, this type of ceramic restoration is very brittle, which makes them suitable only for low-stress-bearing anterior applications. 60 The application of silica-based ceramics as a veneer is made by a mixture of fine glass particles, around 25 µm diameter, with polymeric binders in an aqueous medium to form a powder slurry. 61 This solution can be applied directly on a dental core fabricated by metal, called PFM, or another ceramic core, such as zirconia or lithium disilicate. 62 The porcelain layer is heated slowly to evaporate the binder and to coalesce the particle to form a dense part, and then it is cooled slowly to prevent cracking and crazing. 61 Despite the outstanding customizable aesthetics, the bilayered structure may present, the strong ceramic core coated with porcelain faces some drawbacks regarding properties incompatibility.
Chipping and delamination are the main causes of failure due to, in part, the weak bonding between the core and veneer. 52,63 Usually, the porcelain layer has lower toughness and CTE. 64 In addition, the multiple steps involved in the veneering process develop residual stress that negatively affects the adhesion of the porcelain on the core surface. 65 Hence, an alternative approach is the monolithic ceramic design fabricated directly by CAD/CAM or press technology (e.g., glass ceramics). 66 There are many ways to evaluate the aesthetic aspect of fabricated ceramic teeth. One of them is the visual method, in which the natural tooth is compared with a shade guide provided by the manufacturer.
The most used shade guide is Vita Classical from Vita Zahnfabrik, which presents the appearance of  67 However, each porcelain system can only precisely match in hue with its proprietary guide system. 68,69 Another way to evaluate the aesthetic aspect is by the masking ability of the dental material, which can be defined as the color difference result (ΔE) between the core and the coating. 70 When this value equals zero, the background color is hidden by the outlying structure. A parameter that influences the masking ability is opalescence, which is one of the optical properties that displays the blueness of the reflected light spectrum and brownness-orangeness of the transmitted light spectrum. Opalescence is affected by the material composition, particle size, and thickness of the ceramic. 71 To report the relationship between layer size and optical properties, Valizadeh, et al. 72 (2020) investigated the effect of ceramic thickness on opalescence parameters. The feldspathic ceramic was compared to lithium disilicate and zirconia 3 rd generation. Cylindrical samples of 10 mm diameter with 0.5 and 1.0 mm thick of feldspathic ceramic samples were produced using an aqueous solution of porcelain powder followed by baking under vacuum to a maximum of 920 °C. Lithium disilicate samples were prepared by the wax removable die technique. Lastly, zirconia 3 rd generation was obtained by machining monolithic blocks. A spectroradiometer measured the opalescence in the transmittance and reflectance modes. The effect of ceramic type on opalescence was significant, while the effect of ceramic thickness on opalescence was not. Also, the interaction effect between ceramic type and ceramic thickness on opalescence was significant. The results showed that in all ceramic groups, except for lithium disilicate, the mean opalescence of 1 mm thickness was higher than that of 0.5 mm thickness specimens. This was expected since higher thicknesses allow light transmission through the media resulting in incomplete masking and increased opalescence. Usually, a ceramic restoration comprises an opalescent material, ceramic, an A2 shade, and a masking agent. Thus, the discussion pointed out that the reason for the opalescence increase with higher thickness was the composition of each material. Since lower amounts of masking agent promote a higher share of the opalescent agent in the scattering of blue light, lithium disilicate, which has a low masking agent, presented decreased opalescence with 1 mm thickness. Alternatively, feldspathic ceramic has a limited amount of opalescent material in its composition, making it the most translucent ceramic among the tested groups.
Staining of ceramic restorations is a procedure widely used to mimic the nuances and shades of natural teeth. This occurs because monolithic restorations without subsequent customization after milling do not meet high aesthetic demands.
Unfortunately, the stain layer is removed by wear processes. To understand which ceramic material allows the maintenance of the staining layer for a more extended period, Dal Piva, et al. 73 (2021)  Silica-based ceramics coating can create definitive restorations with individualized and natural optical characteristics. In contrast, this type of ceramic presents a lower survival rate that limits its application to the anterior region. Thus, silica-based ceramics are suitable for single-tooth restorations, such as veneers, inlays, and onlays.

Leucite reinforced feldspathic ceramics
Leucite is a potassium aluminum silicate with a composition of tetragonal KAlSi 2 O 6 , morphology of lamina-like crystals, and size from 1 to 5 µm. 74 At high temperatures, leucite exists as a cubic polymorph. 75 Leucite-based glass-ceramic can be commercially found as IPS Empress® and Finesse®, among others, usually manufactured by hot press technology or CAD/

CAM. 74
Leucite crystals reinforce the glass by restricting In conclusion, the leucite-reinforced feldspathic ceramic has higher CTE values. This property decreases the thermal mismatch between the ceramic and the metal when using the PFM configuration.
Leucite-reinforced porcelain also has a CAD/CAM block version.

Lithium disilicate ceramics
Lithium disilicate (LDS) is a glass-ceramic widely used in dental applications due to its good mechanical properties, biocompatibility, and aesthetic performance. LDS is composed of 65 vol% of lithium disilicate (Li 2 Si 2 O 5 ) with small needle-shaped crystals embedded in a glass matrix with 1 vol% porosity.
These features make LDS one of the most popular all-ceramic materials for dental restorations. 80 The good mechanical properties of this glassceramic are due to two major factors: 1) their elongated disilicate crystals form an interlocking pattern, which hinders crack propagation, and 2) the divergence between the thermal expansion coefficients Long-term (10 to 16.9 years) clinical retrospective data on the performance of LDS inlays and onlays, crowns, and partial crowns as a function of multiple variables have been published, and cumulative survival rates are all above 90%. 87,88 However, although high survival rates, comparable to PFM, were observed for LDS monolithic FDP at 10 years (87.9%), an impressive decrease to only 48.6% in survival was reported at the 15-year follow-up, suggesting a decisive role of fatigue and crack propagation over time. 89,90 Due to its good mechanical, optical, aesthetics, and biological properties, combined with reduced thickness, favorable wear behavior, and minimally invasive approach, LDS is currently one of the most popular metal-free materials for dental restorations.
New lithium disilicate materials have been launched in the market with essential innovations in the glass composition and crystal structure. 91 However, some drawbacks can limit their use in dental applications.
A different approach has been developed to overcome these limitations.

Zirconia-based ceramics
Zirconium dioxide (ZrO 2 ), also known as zirconia, has been the most promising material in dental restorations with several advantages, including superior flexural strength, K Ic , biocompatibility, biofunctionality, and affordability. 103 Advanced zirconiabased ceramics are widely used in oral rehabilitation as prostheses to replace the unit, and partial and total absences on teeth and implants. 36 The current development of a novel ZrO 2 generation, which we call the 4 th generation, comprises a multilayer system. This new generation was developed in an attempt to mimic the gradual change of color and translucency of the natural teeth without compromising the mechanical properties. The multilayer system works with different amounts of cubic phase in each layer. 115 However, this phase is also known for its lower mechanical properties compared to the tetragonal phase. Thus, the challenge now is to understand the mechanical behavior of this new system by studying the complex interface between the layers. Table 3 shows the composition, material properties, and application of each generation of zirconia.
Due to the large selection of zirconia materials, it is difficult for dentists to evaluate and choose the most suitable generation for each specific treatment case. When it comes to the 3 rd and 4 th generations, the scientific data comparing each other is scarce, mainly because the multilayer system is the latest released on In-Ceram Zirconia was better suited this purpose,  Although some commercial zirconia has been used to manufacture monolithic crowns, some of these materials present poor translucency due to their high

Conclusions
This review pointed out the last development of restorative ceramic materials, including all-ceramic and metal-ceramic types. Their applications show the current advantages and limitations yet to overcome.
As seen from PFM to multilayer zirconia, no material can fulfill all the needs existing in clinical situations.
The current need is to reach a balance between good mechanical properties and high-quality aesthetic finishing aiming to mimic the optical aspect of natural teeth. Also, considering the novelty of zirconia Y-TZP and ATZ, the reliability of these materials requires further laboratory and clinical investigation.